On 9 May 1976, an Imperial Iranian Air Force Boeing 747-131 crashed as it approached Madrid, Spain. Witnesses observed lightning strike the aircraft followed by a fire, explosion, and separation of the left wing. This event is notable because it is the only accident involving a 747 that has been both thoroughly investigated and in which an explosion was determined to have led to a catastrophic failure of a wing in flight. In National Transportation Report number NTSB-AAR-78-12 dated 6 October 1978, the Board developed two hypotheses as to what caused the explosion and subsequent wing failure.

The aircraft was purchased from Trans World Airlines, converted to a freighter, and delivered to the Imperial Iranian Air Force on 1 March 1976. Coincidentally, the 747 involved in the TWA event was leased to the Iranian Air Force, but was involved in no unusual incidents prior to its return to the TWA fleet. The flight in question was a military logistic flight from Iran to the U.S. with an en route stop in Spain. The aircraft had been maneuvering around thunderstorm activity in the area and was in contact with approach control. When the aircraft was about 6,000 feet above the ground, witnesses reported that the aircraft was struck by lightning between the left wing tip and engine one, an observed fire and an explosion. This was followed by the in flight breakup of the aircraft. The 10 crew members and 7 passengers were all killed.

Because the accident involved a non-U.S. aircraft outside of U.S. airspace, the National Transportation Safety Board did not have statutory authority to investigate this accident. However, because of the nature of the accident and because the aircraft was a type used extensively in commercial airline operations, they requested and were granted permission to assist in the investigation. Because they did not have any statutory authority, the Board did not determine a probable cause or identify causal and contributing factors in this accident.

After an extensive investigation involving the NTSB and 48 industry and government aviation specialists, the NTSB concluded that there were two possible explanations:

The most probable sequence of events was an ignition of fuel vapors in the number one fuel tank followed by oscillations of the outer wing, multiple structural failures, and a subsequent separation of the left wing. Damage in the area of the tank provided positive indications of an explosion.

The evidence of a lightning strike an instant before the explosion made the lighting strike a plausible source of ignition. Further analysis pointed to a spark at the motor operated fuel valve in the number one fuel tank as an ignition source that would support the hypothesis that ignition of fuel vapor in the number one fuel tank as the first destructive event.

The next most likely sequence of events was determined to be a structural failure caused by a high velocity gusts and turbulence followed by an explosion in the number one fuel tank. However, the available evidence and the probabilities of an aircraft encountering these unique environmental conditions made this hypothesis less supportable.

An exhaustive examination of the cockpit voice recorder tape did not reveal evidence of any turbulence prior to the lightning strike. Also, evidence from inside the fuel tank did not show any evidence of sloshing fuel at the time of ignition - another sign that there was no turbulence at the time of the explosion. Also an examination of the engine mount fuses and wreckage trail also supported the idea that there were no gusting winds in the vicinity of the aircraft at the time of the breakup. An analysis by NASA did conclude however, that turbulence alone could impose loads that exceed the ultimate design loads of the airplane structure.

Note: This aircraft was fueled using a combination of Jet A and JP-4 fuels. JP-4 is more easily ignited than Jet A and is not commonly used by commercial airliners. The TWA Flight 800 aircraft was fueled only with Jet A.

From the time of this 1976 event until June of 1996, there was no other known case of a 747 or any other large commercial jet airliner that sustained an explosion in or near a wing followed by a separation of the wing from the aircraft. As of 27 July 1996, the NTSB reports that a wing separated in flight during the 17 July 1996 accident involving a TWA 747. However, the NTSB has not made any official comment about the relevance of the 1976 event to the 1996 event.

The proliferation of portable, shoulder fired surface to air missiles has for many years led to fears that an individual or group would attempt to use such a missile to bring down a large commercial jet airliner. For example, according to Jane's Weapon Systems 1988-89, there is a variation of the SA-7 Grail, an infra-red homing missile produced by the former Soviet Union and several other countries, which has a maximum altitude of about 14,500 feet (4500 m). It is used by military forces around the world in countries such as Iran, North Korea, Libya, and Sudan. The missile is not large, with a diameter of about four inches (10 cm) and with a launch weight of about 22 pounds (9.97 kg). This type of missile had been used to shoot down a propeller driven Viscount airliner in Rhodesia 1978.

Jin-Tai Choi in the book Aviation Terrorism mentions one documented case of an SA-7 striking an Omani fighter aircraft at an altitude of 11,500 feet. He also refers to a September 1973 arrest in Italy of five persons armed with SA-7s who had rented an apartment under the flight path to Rome's Fumicimino Airport. They had planned to use the missiles to shoot down an El Al airliner approaching the airport. In that case, the SA-7s had been supplied to Egypt by the Soviet Union.

In the March 1994 issue of Air Transport World, the magazine reported on an October 1993 conference on terrorists threats to civil aircraft that was sponsored by the American Defense Preparedness Association. At that conference, the FAA discussed 23 attacks by surface to air missiles against civil aircraft, about half of which were successful. All of these attacks had taken place in regions of active conflict. One conference attendee admitted that if terrorists targeted the air transport industry with surface to air missiles, that the industry could not institute effective countermeasures for five to ten years.

Should airlines have to actively counter this missile threat, there would be few effective countermeasures available. Military tactics such as tight, spiral turns into or out of airports, decoy flares, and severely restricting access within several miles of airports would be difficult or impossible to impose in the civilian world. There are other measures such as diffusing engine exhaust and using infra-red suppressing paint which would provide some level of protection but at a substantial costs to airlines. It is very likely that no serious efforts would be made to institute significant countermeasure strategies unless there were a significant and successful attack against some large jet airliner.

During a November 1995 airport security seminar organized by Todd Curtis of AirSafe.com, the seminar observed the security procedures at the Fort Lauderdale, FL airport.
The intention of the group was to act like a terrorist group looking for targets of opportunity.
What the seminar discovered was at times fascinating and at other times frightening.

In general U.S. airports have two areas where passengers and visitors have access: a public area with little active security measures and a more secure area in the aircraft waiting and boarding areas. The less secure areas usually contain ticket counters, baggage claim, gift shops, restaurants, and other airport services. Getting into the gate areas involve going through a screening process that includes x-ray inspection of carry on items and walking through metal detectors. Other security measures include limiting curbside parking at the terminal, securing unattended luggage, and requiring that all passengers be identified by the airlines by use of a picture identification. In the academic exercise, the group made several notable security observations.

Trash Bins: Most of the trash bins in the terminal area were set within larger concrete containers. An explosive set within one of these containers would likely be directed upward. However, in several cases there were metal and fiberglass containers, sometimes adjacent to the concrete ones, were also located around the terminal.

Unattended Baggage: During the visit, there were numerous announcements about how unattended baggage would be collected by the airport authority. At one point, one of the participants observed an unattended umbrella propped against a wall near one of the screening areas. The umbrella was in plain view and in close proximity to constant foot traffic. It was over 45 minutes before an airport staff member removed the umbrella

Most areas of the terminal were designed such that it was difficult to leave a bag unattended in a heavily traveled area of the terminal without it being seen. However, a large restaurant in the non-secure area of the terminal had a number of places in the facility where a large suitcase could have been left without being seen unless one were to walk very close to the bag. The implications of this restaurant design were quite obvious to all those in the seminar.

Airport Staff: The group specifically observed custodial staff going about their duties to see if they were security conscious. We were impressed with their thoroughness. Many hidden places such as bathroom trash containers, bathroom stalls, and areas behind furniture or machinery were visually checked by the custodial staff.

Treatment of the Group: At one point, different members of the group sat or stood in an area directly behind the staff at one security checkpoint to observe the x-ray monitor and general security procedures. Two females from seminar were able to observe for as long as they wanted and were not approached by airport or security staff.
A male member of the group, who was also a police officer, was told to move along soon after he arrived.

Identifying Passengers: Three of our party checked in for flights that day on two different airlines. At the time, all passengers were supposed to be identified with a picture identification. I was never asked for mine, another person used an ID with a name that did not match that on her ticket, and the third was asked for ID, but did not have to open up an oddly shaped package after being asked by a gate agent about its contents.

In general, we were quite impressed with the level of security. The most worrisome aspects of what we saw were that the effectiveness of active and passive security measures varied greatly, and that a group of people unschooled in the ways of terrorism could very quickly discover numerous opportunities for committing mayhem without being detected.